House flippers, homebuilders, and homeowners alike know that an updated bathroom adds great value to a home. Toilets get quite dirty and leak over time and it may be time to replace them. When installing a toilet it is important to know what size drain pipe to use.
The optimal drain pipe size for a toilet is a diameter of 3 or 4 inches (7.62 – 10.16 cm/76.2 – 101.6 mm). A wider pipe can move more waste and is less likely to clog.
Read on to learn more about the sizing of pipes, installation considerations, explore some FAQs, and more.
Main Topics
3-inch vs. 4-inch Pipes
A 4-inch pipe can drain about twice as much as a three-inch pipe. This size is also better suited for handling waste from more than one toilet. Both a 4-inch and 3-inch drain can be installed leading from the toilet to connect the main 4-inch drain pipe that is part of a home’s septic or sewer system.
Generally, older homes will have 3-inch pipes, meaning that clogs can happen more easily if there is more than one toilet hooked up. Newer construction will generally use 4-inch pipes. Today, toilet pipes are made of copper or PVC. Some homes built before the 1960s may have iron or steel.
Toilet Drain Pipe Size Considerations
The age of a home and its pipes, as well as the sizing of the toilet’s components, can affect the pipe size. Toilets have a part called a flange that attaches to a toilet, through the floor, to the drain line. If the toilet has a 4-inch connection to a 3-inch pipe, an accommodating flange will be needed to make sure the toilet drains properly and is securely connected.
If possible, a home with multiple toilets should use 4-inch pipes to reduce the chances of clogging. This may not be feasible for homes that are not new builds, and most homeowners will find that they have 3-inch waste pipes.
Your state may have a toilet drain size code, however, you will find that the minimum drain size is going to be 3 inches. Pipes need to be this size to accommodate waste and volume as the toilet is flushed.
Measuring Drain Pipe
PVC pipes from a hardware store will have markings on them to indicate the size either in fraction or decimal form. However, if you do not see the size of any pipe, you can measure the outside diameter (OD) with a flexible measuring tape.
To do this, measure around the circumference and divide it by pi (π) including the first couple of decimal digits, 3.14.
So, for example, a pipe with a circumference of 11.0 inches divided by pi equals 3.5. This final answer is called a nominal pipe size (NPS). This is indicative of the inside diameter for managing flow.
Here is a brief table to show some OD and NPS examples in inches:
| OD (Outside Diameter) | NPS (Nominal Pipe Size) |
|---|---|
| 0.84” | 0.5” |
| 1.315” | 1.0” |
| 2.375” | 2.0” |
| 3.5” | 3.0” |
Other Pipe Considerations
Slope
Toilet waste pipes should also flow downhill into the main waste line. Ideally, the slope of the pipe from the toilet should be a ¼-inch drop per foot of horizontal pipe. Using gravity and a downward push will help move waste along.
Water pressure will decrease at a rate of one half-pound per square inch per foot of piping above the water supply. Clean water that flows in may be of lesser pressure the higher up the pipes are in your home.
Entry Points
If there are multiple toilets on a waste line, the piping should have separate points of entry into the main line or else waste can come back up. For new construction, using 4-inch pipes will also reduce the chances of this happening.
Venting
Vented pipes will keep the odor from sewer and wastewater gasses out of your home controlling the air pressure within the pipes. If pipes are not vented properly, then drains could overflow or the toilet could back up.
Length of Toilet Waste Pipe
The maximum length of the toilet waste pipe depends upon the diameter of the pipe. All wastewater flows to the main stack of your plumbing system. This stack is where all of the plumbing fixtures lead to and out of the home into a septic or sewer system.
If you are using a 4-inch pipe then the toilet pipe can be up to ten feet from the stack where it connects to the main line. These distances are important to allow waste to flow instead of clogging. If the drain pipe is three inches in diameter, then you can extend out six feet from the stack.
Plumbing A Bathroom
Here’s a video on how to plumb a bathroom:
This video also includes toilet pipe diagrams in much detail.
Unblocking A Drain Pipe
If your plumbing is prone to blocks and clogs you can try a few remedies at home before calling a plumber or using a pipe snake. Keep in mind that lots of very hot water over time can loosen joints on PVC pipe glue, so use with caution.
Hot Water, Baking Soda, Vinegar
Pour boiling water down the drain. Then add 1-cup of baking soda followed by one cup of vinegar. Let it sit for thirty to sixty minutes and then add more hot water to hopefully break up and remove the blockage. Repeat as needed until water flowers freely.
This method also helps eliminate hair clogs and soap scum residue. Boiling water is best to use with metal pipes, otherwise use your hottest tap water.
Hot Water and Salt
Pour one half of a cup of table salt into the drain, then pour in some boiling hot water. The hot water will soften blockages and the salt will scrub the inside of the pipes.
Plunger
Use a plunger if the toilet is clogged. The force of the suctioning can help push clogs out of the way into the larger connecting pipes.
Chemical Cleaner
Use a chemical cleaner formulated for use in pipes if the more natural ways are not working. Make sure you read the instruction label for use and handling.
Soda/Pop
Try Coke or Pepsi which has phosphoric acid in them to break down buildup and remove limescale.
Plumber
If you cannot get the water to flow properly, you will need some professional help. Remember that you should not use boiling hot water with PVC pipes as this can loosen joints over time, resulting in the need for replacement pipes and possibly a plumber.
If you have slow-moving drains or notice gurgling sounds throughout the entire house’s plumbing, then your main line may be clogged and the sewer lines are backed up. This means you will need to call in a plumber to clear the entire system out.
FAQs
Can you use a 2-inch pipe for a toilet?
You should not use a 2-inch pipe for a toilet. If the toilet pipe size for the drain is two inches (5.08 cm/50.8 mm), or smaller, then your toilet was installed on a plumbing line designed for a sink, shower, laundry sink, or washing machine. Therefore, clogging is going to occur much more often. You may need a special macerating toilet to grind up toilet waste in these cases.
While you can use a 2-inch pipe for a toilet, it is not the best option. A minimum of 3-inches is used for toilet waste.
Can showers and toilets share a drain pipe?
In most areas of the US, showers and toilets can share a drain pipe.
The wastewater from each will end up in the same sewer line. However, they should not share a waste arm trap to ensure that your shower remains a sanitary place to bathe.
Can the shower be used as a toilet?
A shower drain cannot handle solid waste material, and should not be used as a toilet.
First of all, as you read above, a shower drain pipe is generally two inches in diameter.
Secondly, there is no flushing force of motion combined with a large amount of water to move waste along.
You will likely end up with an unsanitary clog in your shower if you put solid waste into the shower drain pipe.
Key Takeaways
Install a toilet with a 3- or 4-inch drain pipe. These standard sizes allow waste to flow properly to the main plumbing line and out of your home. If you have multiple toilets on a line, 4-inch is best to avoid clogging issues.
